Om
oṃ · praṇavaThe syllable chanted at the start or end of many classes. The texts call it praṇava.
The Yoga Sutras introduce Om not as a mood-setter but as a name, the sound-form of īśvara, the lord the text has just described:
The word-expressing Him is the Mystic-syllable (pranava).
The original Sanskrit
tasya vācakaḥ praṇavaḥ
And the very next line says what to do with it, which is the oldest chanting instruction on this site:
The repetition of this (Om) and meditating on its meaning (is the way).
The original Sanskrit
tajjapas tadarthabhāvanam
Repeat it, and reflect on what it means. Whatever your relationship to the metaphysics, that is a complete practice in eight words.